A particle on the classical view is a concentration of energy and other properties in space and time, whereas a wave is spread out over a larger region of space and time. The question whether light are streams of particles (corpuscles) or waves is a very old one. This "either - or" formulation was classically natural and alien to the advanced "both - and" even the "neither - nor" solution of today. Early in the nineteenth century experiments were suggested and made to show that light is a wave motion. A key figure in this endeavor was Thomas Young, one of the most intelligent and clever scientists ever to live, who studied diffraction and interference of light already in 1803 with results that gave strong support to the wave theory of Christian Huygens as opposed to the particle or corpuscular theory of Isaac Newton. Further contributions were made by many other researchers, among them Augustin Jean Fresnel, who showed that light is a transverse wave.

•Newton's theory of light had seemed suitable to explain the straight-line casting of sharp shadows of objects placed in a light beam. But wave theory was needed to explain interference where the light intensity can be enhanced in some places and diminished in other places behind a screen with a slit or several slits. The wave theory is also able to account for the fact that the edges of a shadow are not quite sharp.

The mathematical theory of electromagnetism by James Clerk Maxwell, set up in 1864, led to the view that light is of electromagnetic nature, propagating as a wave from the source to the receiver. Heinrich Hertz discovered experimentally the existence of electromagnetic waves at radio-frequencies in the 1880s. Maxwell died in 1879 and Hertz died only 37 years old in 1894, two years before Alfred Nobel's death.

At the end of the 19th century, which also is the time when the Nobel Prizes were instituted, the wave nature of light seemed definitely established. Thus the decisive research into the wave nature of light came too early to be considered for Nobel Prizes. However, there is one exception - the case of X-rays.

Discoveries relating to the particle nature of light belong to our century and thus one might expect Nobel Prizes be awarded for such achievements. This is almost true - but the Nobel archive tells a more complicated story as will be uncovered below.

• Light is a transverse, electromagnetic wave that can be seen by humans.
  • The wave nature of light was first illustrated through experiments on diffraction and interference.
  • Like all electromagnetic waves, light can travel through a vacuum.
  • The transverse nature of light can be demonstrated through polarization.
  • Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light".
  • Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light"
• Light is produced by one of two methods.
  • Incandescence is the emission of light from "hot" matter (T ≳ 800 K).
  • Luminescence is the emission of light when bound electrons fall to lower energy levels.
• The speed of light depends upon the medium through which it travels.
  • The speed of light in a vacuum is a universal constant in all reference frames.
  • All electromagnetic waves propagate at the speed of light in a vacuum.
  • The speed of light in a medium is always slower the speed of light in a vacuum.
    (The difference is usually negligible when the medium is air.)
  • The speed of anything with mass is always less than the speed of light in a vacuum.
    (The speed of light in a vacuum is the universal speed limit.)
  • The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
• The amplitude of a light wave is related to its intensity.
  • Intensity is the absolute measure of a lightwave's power density.
  • Brightness is the relative intensity as perceived by the average human eye.
• The frequency of a light wave is related to its color.
  • Color is such a complex topic that it has its own section in this book.
  • Monochromatic light can be described by only one frequency.
    • Laser light is very nearly monochromatic.
    • There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.
  • Polychromatic light is composed of multiple frequencies.
    • Every light source is essentially polychromatic.
    • White light is very polychromatic.
  • A graph of relative intensity vs. frequency is called a spectrum (plural: spectra).
    Although frequently associated with light, the term can be applied to many phenomena.
    • A continuous spectrum is one in which every frequency is present within some range.
      • Blackbody radiators emit a continuous spectrum.
    • A discrete spectrum is one in which only a set of well defined and isolated frequencies are present.
      (A discrete spectrum is a finite collection of monochromatic light waves.)
      • The excited electrons in a gas emit a discrete spectrum.

  condition	description	spectrum
  hotter than red hot	incandescent	continuous
  excited electrons	luminous	discrete

• The wavelength of a light wave is inversely proportional to its frequency.
  • Light is often described by it's wavelength in a vacuum.
  • Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.
    • Wavelengths slightly shorter than 400 nm are said to be ultraviolet.
      (They are "beyond violet" in terms of frequency.)
    • Wavelengths slightly longer than 700 nm are said to be infrared.
      (They are "below red" in terms of frequency.)
• Phase differences between light waves can produce visible interference effects.
  (There are several sections in this book on interference phenomena and light.)